In high combustion temperatures, nitrogen oxides (NOx) are formed in the piston engine's cylinder, which are released to the air with the exhaust gases. Because of the environmentally detrimental effects of the nitrogen oxide emissions, efforts have been made to minimise them.
Adding water with the combustion air to the combustion process reduces, as is well known, the generation of nitrogen oxide emissions. This phenomenon is based, among other things, on the cooling effect of the water. In practice, adding water to the piston engine's combustion process has been put into practice mainly in three different alternative ways so that water is fed either directly to the engine's combustion chamber, water is fed with the fuel as a fuel emulsion or the feeding of the water is brought about through a suction air channel to the suction air.
In publication U.S. Pat. No. 5,758,606, a method of feeding water to the engine's suction air subsequent to the supercharger has been presented. The water is first heated with the engine's cooling water and fed into a separate humidifying tower, in which the water sprayed into mist evaporates. This kind of arrangement is impractical e.g. because of the space the humidifying tower requires and it brings about a certain security risk as the large-size humidifying tower operates under high pressure. Also documents EP-A-0916836, U.S. Pat. No. 4,960,080, EP-A-1076169 and U.S. Pat. No. 6,082,311 disclose a supercharged engine, wherein water content of the combustion air is increased before bringing the air into the combustion chamber.
Feeding water with the suction air to the combustion space does not, as such, decrease the shaft efficiency of the engine, but in certain conditions the temperature of the suction air would have to be raised high, so that the air humidity could reach, from the point of view of reducing the nitrogen oxide emissions, a sufficient level, which in turn reduces the mass flow rate and the oxygen content of the air. The amount of water obtained evaporated in the combustion space can therefore only be the amount, which remains in gaseous form in suction air's pressure- and temperature conditions. Therefore, the amount of water obtained in the combustion space with the suction air is limited by the saturation of water vapour in the conditions of the suction air. The amount of water needed is relatively high because, due to the continuous humidification of the suction air, part of the water cannot be exploited because it exits during the cylinder's scavenging stage, in which case both the suction valve and the exhaust valve are open at the same time.
In publication EP 0 683 307 A1, an equipment for feeding water directly to the engine's cylinders has been presented depending on the firing order of the engine. Water is sprayed during the suction stroke controlled by the regulating unit, which uses the engine's rotation speed, the position of the piston and/or the engine's operating conditions as input data. In this kind of solution, the problem is e.g. the optimal water distribution to the combustion space, so that too much water doesn't have to be sprayed and, on the other hand so that the desired effect is obtained. In addition, the feeding of the water directly to the combustion space requires a rather complicated equipment. The feeding of the water and the fuel can also be carried out alternately with the same nozzle arrangement.
Feeding water directly to the combustion space is known as such also by feeding water with the fuel as an emulsion. This solution is advantageous in many ways, but its particular problem is that the equipment has to be dimensioned, determined by the combined fuel and water flow rate, relatively high in its capacity. In this case, especially with partial load, the operation is not quite optimal. In publication EP 0 742 363, a solution has been presented to the problem of conventional fuel-water-emulsion injection. Used as such, the preparation systems of the fuel-water-emulsion can be relatively complicated and the control of the water amount thereof is slow.
It is an object of the invention to provide an improved method and arrangement of reducing nitrogen oxide emissions and also smoke formation in a supercharged piston engine, which is based on taking advantage of water in the combustion process, but of which the defects of the prior art techniques have been essentially eliminated. To be more precise, it is an object of the invention to provide a method and an arrangement in which water-fuel emulsion is used as is shown in publication EP 0 742 363, but in which shortcomings of such system and method are eliminated.